Electromagnetic method and apparatus for the measurement of linear position

Abstract

A method and apparatus are disclosed for monitoring a linear position, such as the distance between a sensing element and a movable object, and/or related parameters, such as displacement, direction, speed, velocity, and/or acceleration. The method utilizes a sensing element and an electrically conductive portion of the movable object, or a conductive target coupled to the movable object. The apparatus includes at least one sensing element formed by a section of a coupled slow-wave structure. The sensing element is connected to an RF or microwave generator, and an electronic circuit that converts at least one electromagnetic parameter of the section of the coupled slow-wave structure into a position reading. Electric and magnetic fields excited in the sensing element are split so that most of the electric energy is concentrated inside of the sensing element, while most of the magnetic energy is concentrated outside of the sensing element. The distance that defines the linear position is measured as a resonant frequency, impedance, phase shift or other electromagnetic parameter.

1. A method for monitoring a position of an object, or related parameters, including a measurement of the position of the object as a distance between the object and a sensing element, motion of the object along the distance defining a motion axis, the method comprising:
 placing at least one electromagnetic sensing element in proximity to the monitored object; 
 exciting an alternating electromagnetic field in the sensing element at a frequency and in a form at which the electromagnetic field extends to the object; 
 measuring a variation of at least one electromagnetic parameter of the sensing element, the variation caused by a variation in the position of the object; 
 converting results of the measurements into a representation of the position of the object along the motion axis, 
 wherein 
 at least one electrically conductive surface is formed on, or coupled to, the monitored object, the surface facing the sensing element, the alternating electromagnetic field exited as an opposite-phase slowed-wave in a coupled slow wave structure, the electromagnetic field having a spatial distribution, the distribution depending on the position of the conductive surface; and 
 providing an indication of the position of the object. 
 
   
   
     2. The method of  claim 1 , wherein:
 a metal wall of the monitored object is used as the conductive surface. 
 
   
   
     3. The method of  claim 1 , wherein:
 the conductive surface is disposed on a target coupled to the object, 
 the target having a physical geometric configuration similar to the sensing element configuration. 
 
   
   
     4. The method of  claim 1 , wherein:
 the opposite-phase electromagnetic wave is excited as an axially symmetric slow wave propagating in an axial direction, 
 the axial direction substantially parallel with the motion axis, the surface positioned in parallel to the axial direction. 
 
   
   
     5. The method of  claim 1 , wherein:
 the opposite-phase slowed-wave is excited as an axially symmetric slowed-wave, propagating in a radial direction, 
 the radial direction being perpendicular to the motion axis, the conductive surface positioned in parallel to the radial direction. 
 
   
   
     6. An apparatus for monitoring a position of an object, including a measurement of the position of the object as a distance between the object and a sensing element, motion of the object along the distance defining a motion axis, the apparatus comprising:
 at least one electromagnetic sensing element; 
 at least one target having an electrically conductive surface, the target located proximate the monitored object; and 
 a measuring circuit connected to the sensing element, the circuit including:
 at least one radio frequency or microwave generator; and 
 a converter convening electromagnetic parameters of the sensing element into an electrical representation of linear position, 
 wherein: 
 
 the sensing element including at least one section of a coupled slow-wave structure, the generator exciting a slowed-wave into the sensing element, the slowed wave propagating in a direction, the section of coupled slow-wave structure having at least two impedance conductors configured as patterns, the patterns fashioned as at least one row of conductive members arranged in series in the direction of the slowed wave propagation and connected to one another with a pitch, h; and 
 the pattern configurations of the impedance conductors being mirror images of one another flipped by approximately 180 degrees with respect to a surface of symmetry of the patterns, the patterns being set to distribute in a given ratio the components of electric and magnetic fields at the conductive surface of the target. 
 
   
   
     7. The apparatus according to  claim 6 , wherein:
 there exists a gap g between the impedance conductors, the slowed-wave having a deceleration in the slow-wave structure and a wave length λ in free space, the gap and wave length of the electromagnetic wave approximately defined by the inequality 
 
     
       
         
           
             
               g 
               < 
               
                 λ 
                 
                   20 
                   ⁢ 
                   π 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   N 
                 
               
             
             , 
           
         
       
     
     where N is the deceleration of the electromagnetic wave in the slow-wave structure. 
   
   
     8. The apparatus according to  claim 6 , wherein:
 the impedance conductors are formed by at least two approximately identical radial spirals that are wound in opposite directions. 
 
   
   
     9. The apparatus according to  claim 8 , wherein:
 at least two of the radial spirals are formed as metallization on at least two sides of a dielectric substrate. 
 
   
   
     10. The apparatus according to  claim 6 , wherein:
 the impedance conductors are formed by at least two coaxially positioned helices that are wound in opposite directions. 
 
   
   
     11. The apparatus according to  claim 10 , wherein:
 at least two of the helices are formed as metallization on at least one dielectric tube, through at least one dielectric layer.